Another great alpha from the Firebug team. See some highlights from this release:

Panel for CSS Selector Queries

Custom Firebug Commands

Improved $ and $$ Commands

Panel for CSS Selector Queries

Firebug offers new side panel (available in the CSS panel) that allows quick execution of CSS selectors. The panel also shows the list of matching elements.

In order to see list of matching elements for an existing CSS rule, just right click on the rule and pick Get Matching Elements item. The list of elements (together with the CSS selector) will be displayed in the side panel.

Of course, you can also use custom CSS selectors. Just click on the yellow edit line at the top of the side panel and type your own selector.

This feature has been previously implemented in a separate SelectBug extension.